Spacecraft such as satellites generally support devices having energy detecting capabilities of one type or another. Typical remote sensing devices provide measurement of reflected (primarily solar) or emitted (from man-made sources) visible and near-infrared energy from the Earth or other heavenly bodies. A method of calibrating the measured radiance from Earth (or another source) is to create a reference radiance using a ubiquitous, known source of spectral irradiance, such as the Sun, as reference input to a diffusive reflector that provides a known radiance to the remote sensing instrument.
The standard radiance value may be created by reflecting known solar spectral irradiance from a diffuser panel toward the remote sensing device during an occasional (non-normal operation) calibration. The remote sensing device output is measured as the device receives the known diffusely reflected energy from the diffuser panel. Using the characteristics of the remote sensing device and a reference view of empty space (i.e. no significant irradiance at the remote sensing device), response of the device to known radiance input is determined. This calibration process provides sufficient information to calculate radiance incident to the remote sensing device during normal operation using the instrument output as it views the Earth or other targets of interest.
For example, imagers and other sensors in earth orbit that measure electromagnetic radiation at wavelengths between 0.3 and 0.7 microns (the visible band), such as the Landsat satellite program, are often calibrated using solar radiation reflected off a diffusely reflective surface, as shown in FIG. 1. The sensor in FIG. 1 is represented schematically by single lens 12 focusing an image onto detector array 14. The sensor is calibrated by swinging diffusely reflecting surface 10, often a lambertian surface, into a blocking position over the sensor aperture. The sun must be at a large angle to the sensor's line of sight, with solar radiation passing across the aperture so that the lambertian surface can fill the sensor's field of view with diffusely reflected sunlight. Between calibration cycles the lambertian surface must swing aside, clearing the line of sight, as shown by the double arrow and dashed lines in FIG. 1.
The drawbacks of the calibration procedure similar to the one shown in FIG. 1 are well known. First, the procedure is not ideal for satellites in orbits where the sun is at small, rather than large, angles to the sensor's line of sight. Second, the space required for the lambertian surface to swing in and out of the line of sight takes up a lot of space on the satellite, creating difficulties for satellite designers. Finally, when the sensor is viewing scenes containing regions where the albedo is very close to one (for example high-altitude cloud tops from earth orbit), it is preferable for the detectors to be calibrated using radiances somewhat greater than those coming from the albedo-one regions. However, even using an ideal and diffusely reflective lambertian surface, i.e. one that does not absorb any visible radiation, the procedure shown in FIG. 1 may only produce radiances as great as, and not greater than, those coming from albedo-one regions. Thus, it is likely that the sensor in FIG. 1 is calibrated at radiance levels somewhat less than the brightest radiances measured while in use.
The present invention addresses a solution to the problem of calibrating an imaging sensor disposed in an orbiting satellite using solar radiation. Advantageously, the present invention reduces the angle at which the sun is relative to the sensor's line of sight, reduces the physical space required on the satellite for the lambertian surface to move into the sensor's line of sight, and provides radiance levels that are greater than the brightest radiance levels measured during a typical operation.